Polymers having reactive terminal silyl groups or compositions comprising such polymers can be hydrolyzed and condensed in the presence of water and metal catalysts. Suitable known catalysts for curable compositions include compounds employing metals such as Sn, Ti, Zn, or Ca. Organotin compounds such as, for example, dibutyltin dilaurate (DBTDL) are widely used as condensation cure catalysts to accelerate the moisture-assisted curing of a number of different polyorganosiloxanes and non-silicone polymers having reactive terminal silyl groups such as room temperature vulcanizing (RTV) formulations including RTV-1 and RTV-2 formulations. Environmental regulatory agencies and directives, however, have increased or are expected to increase restrictions on the use of organotin compounds in formulated products. For example, while formulations with greater than 0.5 wt. % dibutyltin presently require labeling as toxic with reproductive 1B classification, dibutyltin-containing formulations are proposed to be completely phased out in consumer applications during the next four to six years.
The use of alternative organotin compounds such as dioctyltin compounds and dimethyltin compounds can only be considered as a short-term remedial plan, as these organotin compounds may also be regulated in the future. It would be beneficial to identify non-tin metal catalysts that accelerate the condensation curing of moisture-curable silicones and non-silicones. Desirably, substitutes for organotin catalysts should exhibit properties similar to organotin compounds in terms of curing, storage, and appearance. Non-tin catalysts would also desirably initiate the condensation reaction of the selected polymers and complete this reaction upon the surface and may be in the bulk in a desired time schedule. There are therefore many proposals for the replacement of organometallic tin compounds with other metal-based compounds. These other metals have specific advantages and disadvantages in view of replacing tin compounds perfectly. Therefore, there is still a need to address the weaknesses of possible metal compounds as suitable catalysts for condensation cure reactions. The physical properties of uncured and cured compositions also warrant examination, in particular to maintain the ability to adhere onto the surface of several substrates.
The use of bismuth(III) complexes as catalysts in condensation curable silicone compositions has been described. U.S. Publication No. 2003/0069379 claims the use of trivalent bismuth carboxylates as curing catalysts in room-temperature-curing organopolysiloxane compositions. U.S. Publication Nos. 2011/0009558 and 2011/0021684 claims the use of bismuth(III) tris(monoallyl ethylene glycolate) and bismuth(III) tris(1,1,1,5,5,5-hexafluoropentanedionate) as catalysts, respectively, in curable organopolysiloxane compositions. U.S. Pat. No. 7,365,145 generically describes, among others, organobismuth compounds in a generic list of organic dibutyltin, zirconium complex, aluminum chelate, titanic chelate, organic zinc, organic cobalt, organic iron, and organic nickel as catalysts in moisture-curable silylated polymer composition. U.S. Pat. No. 5,194,489 describes the use of bismuth carboxylate as a hardening catalyst for a crosslinkable cyclopentenyl containing diorganopolysiloxane composition, which also comprises an inorganic filler. U.S. Publication No. 2009/0156737 describes, among others, Lewis acid compounds of bismuth in a generic list of Lewis acid compounds of Ti, Zr, Hf, Zn, B, and Al as catalysts in polymer blends comprising fillers and alkoxysilane-terminated polymers. Similar generic descriptions on the use of bismuth carboxylates in curable silicone composition are made in U.S. Publication No. 2009/306307. U.S. Pat. No. 7,504,468 describes the use of a mixture of metal-based compounds that include, among others, bismuth compounds as catalysts in single-component silicone compositions. U.S. Publication No. 2005/0137322 describes the use of a bismuth catalyst in a second component along with a polyol in a two component coating composition comprised of a compound containing trialkoxysilyl and isocyanate functional groups as a first component.
U.S. Pat. No. 4,293,597 includes bismuth salts of mono- or dicarboxylic acids in a generic list of metal salts including Pb, Sn, Zr, Sb, Cd, Ba, Ca, and Ti as catalysts in curable silicone rubber compositions that also contain nitrogen-functional silanes. U.S. Pat. No. 4,461,867 includes bismuth carboxylates in a generic list of metal carboxylates also including Sn, Pb, Zr, Sb, Cd, Ba, Ca, Ti, Mn, Zn, Cr, Co, Ni, Al, Ga, and Ge as catalysts in moisture-curable RTV-1 silicone compositions. U.S. Pub. No. 2011/0098420 includes, among others, bismuth compounds in a generic list also including compounds of Pt, Pd, Pb, Sn, Zn, Ti, and Zr, as dehydrogenative condensation reaction catalysts for a curable polysiloxane composition comprising siloxanes with 2 or more hydrosilyl groups and siloxanes with 2 or more silanol groups. U.S. Pat. No. 7,527,838 describes, among others, bismuth-based catalysts in a generic list that includes other metal catalysts based on Sn, Ti, Zr, Pb, Co, Sb, Mn, and Zn, in curable diorganopolysiloxane compositions used for making insulated glass units. U.S. Pub. No. 2011/0040033 describes the use of a commercially available bismuth triflate catalyst, among other metal triflates based on Sc, Yb, Cu, and Ag.
Despite these generic descriptions that group bismuth complexes together with other metal catalysts, there has not been provided any teachings or catalyst compositions that differentiate the catalytic activity exhibited by different bismuth complexes. Further, there has not been a replacement catalyst for organotin compounds that maintains its ability to cure when exposed to humidity or ambient air, after storage over months in a sealed cartridge. It is always a specific requirement for moisture-curable compositions to achieve the shortest possible curing times, showing a tack-free surface as well as curing through the complete bulk in thick section for RTV-1 and RTV-2 compositions. Additionally, such compositions should provide a reasonable adhesion after cure onto a variety of substrates.
While tin-based compounds are facing regulatory pressures, there are concerns over the toxicology of perfluoroalkyl compounds as well as their bioaccumulation. The U.S. Environmental Protection Agency (EPA) is proposing to tighten regulation of such perfluoroalkyl materials that have the potential of breaking down into toxic perfluoroalkyl carboxylates, such as perfluorooctanoic acid (PFOA), and perfluoroalkyl sulfonates, including perfluorooctanyl sulfonate (PFOS). These substances are expected to bioaccumulate, persist in the environment, and are likely to be “highly toxic”. Also, studies suggest that perfluoroalkyl sulfonates and carboxylates may get released in the air when items made with certain fluoropolymers are burned in municipal waste incinerators. Accordingly, the identification of non-fluorinated, non-tin-based condensation catalysts that can avoid environmental and health concerns is of interest.